Rare earth doped and large effective area optical fibers for fiber lasers and amplifiers

ABSTRACT

Various embodiments described herein include rare earth doped glass compositions that may be used in optical fiber and rods having large core sizes. Such optical fibers and rods may be employed in fiber lasers and amplifiers. The index of refraction of the glass may be substantially uniform and may be close to that of silica in some embodiments. Possible advantages to such features include reduction of formation of additional waveguides within the core, which becomes increasingly a problem with larger core sizes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/789,931, titled “Rare Earth Doped and Large Effective Area OpticalFibers for Fiber Lasers and Amplifiers,” filed May 28, 2010, which is adivision of U.S. patent application Ser. No. 12/246,377, titled “RareEarth Doped and Large Effective Area Optical Fibers for Fiber Lasers andAmplifiers,” filed Oct. 6, 2008, which is a division of U.S. patentapplication Ser. No. 11/693,633 titled “Rare Earth Doped and LargeEffective Area Optical Fibers for Fiber Lasers and Amplifiers” filedMar. 29, 2007, now U.S. Pat. No. 7,450,813, which claims priority toU.S. Provisional Patent Application No. 60/846,012 entitled“Rare-Earth-Doped Large Effective Area Optical Fibers for Fiber Lasersand Amplifiers” filed Sep. 20, 2006; each of the foregoing is herebyincorporated by reference herein in its entirety.

BACKGROUND

1. Field of the Invention

This application relates to optical fibers including, for example, rareearth doped optical fibers and fibers with large effective area, whichcan be used for example in fiber lasers and amplifiers, as well asmethods of making such fibers.

2. Description of Related Art

Fiber lasers have shown great promise over the last decade over theirsolid state counterpart due to a variety of advantages. Fiber lasers areeasy to manufacture, are more efficient in heat dissipation, morestable, produce better beam quality, and are more reliable and compact.

Major limits in scaling up power in fiber lasers include nonlineareffects and optical damage, which are a direct consequence of tightconfinement of the optical mode in the laser. There are a number ofnonlinear effects in optical fibers. Self-phase modulation dominates inhigh peak power ultra short pulse generations. Raman scattering is oneof the major limitations for longer pulses and CW operation. Brillouinscattering dominates in narrow spectral line width application.

There have been many studies on how to counteract these nonlineareffects. Some level of self-phase modulation can be balanced bydispersion in self-similaritons. Raman scattering can be reduced byW-type waveguide design to increase loss at Stoke wavelengths. Brillouinscattering can also be reduced by reducing acoustic waveguiding. Sinceall of these nonlinear effects are a direct consequence of high opticalintensity in the optical fiber core, an increase of core size, which isequivalent to increase of effective mode area, can effectively reduceoptical intensity and consequently nonlinear effects.

Multimode fibers with larger core size can be used to operate as neardiffraction limited amplifiers in the presence of appropriate spatialfilters and/or selective modal excitation of the fundamental mode. Theuse of multimode fibers enables the increase of core size beyond thatoffered by single mode fibers. See, e.g., Fermann et al in U.S. Pat. No.5,818,630, which is incorporated by reference herein in its entirety. Asfiber becomes more multi-mode with an increase of core size, easy launchand robust propagation of fundamental mode are also increasinglyimportant in these fibers in order to maintain good beam quality.

Another effective method of reducing nonlinear effect is to use shortlength of fiber. This approach involves highly rare earth doped hostglass.

What is needed therefore are glasses that provide for large core and/orhigh doping.

SUMMARY OF CERTAIN EMBODIMENTS

Various embodiments described herein include rare earth doped glasscompositions which may be used in optical fiber and rods having largecore sizes. The index of refraction of the glass may be substantiallyuniform and may be close to that of silica in some embodiments. Possibleadvantages to such features include reduction of formation of additionalwaveguides within the core, which becomes increasingly a problem withlarger core sizes.

For example, various embodiments described herein comprise a doped glasscomprising silica having a refractive index, at least about 10 mol %phosphorus in said silica, at least about 10 mol % boron in said silica,and rare earth ions in said silica. The rare earth ions have aconcentration in the silica of at least about 1000 mol ppm. The silicahaving the phosphorus, the boron, and the rare earth ions therein has arefractive index within about ±0.003 or less of the refractive index ofthe silica.

Other embodiments described herein comprise a method of fabricating rareearth ion doped glass. The method comprising stacking multiple rodscomprising rare earth ion doped glass and drawing the stacked rods toform a first rod. In some embodiments, the first rod may be cut intoshorter sections that may be stacked and drawn to form a second rod.This second rod may have an effective refractive index uniformity withless than about 5×10⁻⁴ maximum peak-to-peak variation measured withrefractive index profiler with a spatial resolution of 0.1 μm.

Other embodiments described herein comprise a rod comprising a coredoped with rare earth ions and a cladding. The core has an effectiverefractive index uniformity with less than about 5×10⁻⁴ maximumpeak-to-peak variation measured with refractive index profiler with aspatial resolution of between 0.1 to 0.5 μm.

Other embodiments described herein comprise a fiber comprising a coredoped with rare earth ions and a cladding. The core has an effectiverefractive index uniformity with less than about 5×10⁻⁴ maximumpeak-to-peak variation measured with a refractive index profiler with aspatial resolution of between 0.1 to 0.5 μm.

Other embodiments described herein comprise a rod comprising a coredoped with rare earth ions and a cladding, wherein the core comprises adoped region at least 200 microns square (μm²) with an averagerefractive index within about ±0.003 or less of the refractive index ofthe silica.

Other embodiments described herein comprise a fiber comprising a coredoped with rare earth ions and a cladding, wherein the core comprises adoped region at least 200 microns square (μm²) with an averagerefractive index within about ±0.003 or less of the refractive index ofthe silica.

Other embodiments described herein comprise a step index optical fibercomprising a core having a core radius ρ, a first cladding disposedabout the core, and a second cladding disposed about the first cladding.The first cladding has an outer radius ρ₁. The core and the firstcladding have a difference in index of refraction Δn, and the firstcladding and the second cladding have a difference in index ofrefraction Δn₁. For this step index optical fiber, (i) less than 10modes are supported in the core, (ii) the first cladding radius, ρ₁, isgreater than about 1.1ρ and less than about 2ρ, and (iii) the refractiveindex difference between first cladding and said second cladding, Δn₁,is greater than about 1.5Δn and less than about 50Δn.

Other embodiments described herein comprise an optical fiber system forproviding optical amplification. The optical fiber system comprises anoptical fiber doped with one or more types of rare earth ions. Theoptical fiber has a tapered input and a length extending therefrom. Theoptical fiber system further comprises an optical pump optically coupledto the optical fiber and an optical source optically coupled to thetapered input of the optical fiber. The tapered input end supports areduced number of optical modes than the length extending from thetapered input.

Other embodiments described herein comprise a method of fabricatingglass. The method comprises introducing boron by vapor deposition andintroducing phosphorous by vapor deposition, wherein the boron andphosphorous are introduced at different times. Introducing the boron andphosphorous at different times prevents that reaction of boron andphosphorous in vapor phase.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the apparatus, compositions, and methodsdisclosed herein are illustrated in the accompanying drawings, which arefor illustrative purposes only. The drawings comprise the followingfigures, in which like numerals indicate like parts.

FIG. 1 schematically illustrates a highly doped glass in the form of arod that can be used in fabrication of optical fibers in someembodiments described herein.

FIG. 2A schematically illustrates an apparatus for Modified ChemicalVapor Deposition (MCVD) on a surface of a tube.

FIG. 2B schematically illustrates deposition of core soot in the tubeshown in FIG. 2A.

FIG. 2C is a plot of (a) the refractive index across a core region of aglass perform comprising ytterbium doping, (b) the average refractiveindex of the glass, (c) the refractive index of silica, and (d) anexample of the average refractive index of a conventional silica preformwith similar ytterbium doping level.

FIG. 3A schematically illustrates a preform structure such as can befabricated by the apparatus shown in FIGS. 2A and 2B.

FIG. 3B schematically illustrates a rod fabricated by removing parts ofthe preform of FIG. 3B that do not contain ytterbium.

FIG. 4A schematically illustrates a stack of ytterbium-doped rods andcanning of the stack.

FIG. 4B is a plot of the refractive index profile across the line AA inFIG. 4A.

FIG. 5A schematically illustrates a stack formed by canes produced inthe process shown in FIG. 4A and canning of the stack.

FIG. 5B is a plot of the refractive index profile across the line BB inFIG. 5A, which shows the increased uniformity and refractive index.

FIG. 6 is a photograph of the cross-section of a leakage channel fiberwith an ytterbium-doped core fabricated using techniques such as thosedisclosed above with regard to FIGS. 4A, 4B, 5A, and 5B.

FIG. 7A schematically illustrates the cross-section of a leakage offiber such as shown in FIG. 6.

FIG. 7B schematically illustrates the refractive index profile acrossthe fiber of FIG. 7A formed using ytterbium-doped rods having an averagerefractive index matched to that of silica, which is used to form thecladding.

FIG. 7C schematically illustrates the refractive index profile acrossthe fiber of FIG. 7A formed using ytterbium-doped rods having an averagerefractive index higher than that of silica, which is used to form thecladding.

FIG. 7D schematically illustrates the refractive index profile acrossthe fiber of FIG. 7A formed using ytterbium-doped rods having an averagerefractive index lower than that of the silica, which is used to formthe cladding.

FIG. 8 is a photo of the cross-section of a large core polarizationmaintaining (PM) fiber including an Yb³⁺-doped core.

FIG. 9A schematically illustrates a fiber end facet prepared by splicingwith a coreless fiber.

FIG. 9B schematically illustrates the coreless fiber of FIG. 9A cleavedto form the end facet.

FIG. 10A schematically illustrates a fiber end facet prepared bycollapsing holes in a large core fiber.

FIG. 10B schematically illustrates the coreless fiber of FIG. 10Acleaved to form the end facet.

FIG. 10C schematically illustrates a fiber end facet prepared bytapering and collapsing holes in a large core fiber.

FIG. 10D schematically illustrates the tapered fiber of FIG. 10C cleavedto form the end facet.

FIG. 11 schematically illustrates a step-index fiber designincorporating a rare earth doped core fabricated with a stack-and-drawprocess such as described herein with an average refractive index higherthan that of the silica cladding.

FIG. 12 schematically illustrates a polarization maintaining (PM)step-index fiber design incorporating a doped core fabricated with astack-and-draw process such as described herein with an averagerefractive index higher than that of the silica cladding.

FIG. 13 schematically illustrates a bent resistant fiber designincorporating a doped core fabricated a stack-and-draw process such asdescribed herein.

FIG. 14 schematically illustrates a double cladding fiber formed by aprocess described herein.

DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS

The details of the inventions will be explained through examples andillustrations. It is easy to see by a person skilled in the art thatmany possible variations are also possible, not limited to the processdetails used in the examples.

As described above, one limitation to scaling up power in fiber lasersincludes nonlinear effects and optical damage, which are a directconsequence of tight confinement of the optical mode in the laser. Sincethese nonlinear effects are a direct consequence of high opticalintensity in the optical fiber core, an increase of core size, which isequivalent to increase of effective mode area, can effectively reduceoptical intensity and consequently the nonlinear effects.

The use of multimode fibers can enable the increase of core size beyondthat offered by single mode fibers. Multimode fibers with larger coresize can be used to operate as near diffraction limited amplifiers inthe presence of appropriate spatial filters and/or selective modalexcitation of the fundamental mode. See, e.g., Fermann et al in U.S.Pat. No. 5,818,630, which is incorporated by reference herein in itsentirety.

Conventional fiber reaches a core diameter limit of ˜35 μm if robustfundamental mode operation is required, mainly due to launch difficultyand inter-modal coupling in a highly multi-mode fiber. In the past fewyears, a new design based on photonic crystal fibers (PCF) has beenstudied. This design enables demonstration of large core fiber whichsupports a single mode or very few modes at the expense of a weakwaveguide design.

Limpert et al discloses photonic crystal fibers with core diameter of˜28 μm in “High-power air-clad large-mode-area photonic crystal fiberlaser” in Optics Express, vol. 11, pp. 818-823, 2003, which isincorporated by reference herein in its entirety. This fiber design hasa large number of small holes with hole-diameter-to-hole-spacing ratiod/Λ of less than 0.18 to ensure single mode propagation. The holediameter d used is comparable to the wavelength of operation λ withd/λ≈2. At this small hole size over wavelength ratio, significantoptical power penetration into the air holes takes place and this leadsto severe bending sensitivity. This design is again limited to ˜35 μmfor robust fundamental mode operation, mainly due to high bend loss.

Limpert et al discloses a rod with a photonic crystal fiber design in“Extended single-mode photonic crystal fiber lasers,” Optics Express,vol. 14, pp. 2715-2720, 2006, which is incorporated by reference hereinin its entirety. Core diameters up to 100 μm with photonic crystaldesigns have been achieved in rods with diameter of 1.5 mm. The d/Λratio in 100 μm core rod is 0.2. The rod structure keeps the waveguidestraight and mitigates the high bending loss. Length of the rod islimited to 0.5 meters for practical reasons.

In contrast, the design approach disclosed in U.S. patent applicationSer. No. 10/844,943, entitled “Large Core Holey Fibers,” which isincorporated by reference herein in its entirety, uses a much smallernumber of much larger holes. The design creates much larger leakagechannels for higher order modes to escape, leading to high propagationloss for the higher order modes. The use of much larger air holesreduces bending sensitivity. In a recent implementation of the design,d/Λ as high as 0.65 are used leading to a much reduced bending loss. Themuch larger hole-diameter-to-wavelength ratio d/Λ effectively preventsoptical power penetration into the hole, leading to much improvedoptical guidance over a curved fiber. This type of design also reducesinter-modal coupling due to high leakage loss of the higher order modes,leading to much improved single mode propagation.

To make practical amplifier fibers with the photonic crystal fiberdesign and leakage channel design disclosed in U.S. patent applicationSer. No. 10/844,943, refractive index uniformity over the doped core ispreferably better than what conventional optical fiber fabrication canprovide.

When the core becomes larger, index non-uniformity over the core canlead to formation of local waveguides due to local higher refractiveindex surrounded by lower refractive index material. The formation oflocal waveguides depends on the level of the refractive indexnon-uniformity as well as the geometric size of the indexnon-uniformity. Substantial refractive uniformity over the core canreduce this local waveguiding. In particular, by keeping thenon-uniformity at a small geometric size, local variations in refractiveindex will not form waveguides. In various embodiments, therefore theindex non-uniformity is kept below few times the scale of a wavelength.Additionally, large index non-uniformity is made smaller. Suchuniformity is easier to achieve for small cores than for large cores.

Additionally, waveguiding may be formed in the core if the refractiveindex in the doped region of the core is higher than the undoped regionsof the core. The core may for example, comprise silica. If the portionsof the silica that are doped to provide optical gain have a higher indexof refraction than undoped silica, additional local waveguiding mayresult. Precise index-matching of the doped and undoped regions of thecore may reduce this problem. When the doped area is larger, for examplewith a larger effect core area, closer index matching may be more usefulthan if the doped region is small. Accordingly, in various embodimentdescribed herein, the average refractive index of the doped core also iscontrolled to be close to that of the cladding glass, which is also usedin the core. Such control of the refractive index of the doped glass canbe difficult using known host materials for rare earth ions with dopinglevels of 3000 mol ppm and beyond.

In certain embodiments described herein, the desired refractive indexuniformity and average refractive index can be provided by stackingfluorine-doped silica rods with lower refractive index than silica inaddition to the highly ytterbium-doped rods with higher refractive indexin the core and drawing the stacked rods down to produce a core rod.See, e.g., “Extended single-mode photonic crystal fiber lasers,” OpticsExpress, vol. 14, pp. 2715-2720, 2006, which is incorporated byreference herein in its entirety. The number of fluorine-doped rods andthe number of ytterbium-doped rods are carefully chosen to give anoverall average refractive index close to that of silica. The totalnumber of the two types of rods is large and they are evenly distributedin a stack. The rods are typically stacked into a hexagonal stack anddrawn down to produce a core rod. High degree of average refractiveindex control can be achieved this way. Since light cannot see structurewith dimension much less than the wavelength of the light, highlyuniform refractive index can be achieved when the initial individualrods are reduced to a dimension around the wavelength of the light inthe final core. This technique, however, reduces the level of averageytterbium doping in the final core due to the dilution from thenon-ytterbium-containing fluorine-doped silica rods. Such fluorine dopedrods may be used to achieve average refractive index, but may notcontain ytterbium. This approach will lower the average ytterbium levelwhich is used for providing gain.

Nevertheless, in theory precise control of average refractive index canbe achieved by controlling the ratio of the numbers of rods of each typein a glass comprising two types of rods, one with high refractive indexand one with low refractive index. See, e.g., U.S. Pat. No. 6,711,918 B1issued to Kliner et al, which is incorporated by reference herein in itsentirety. A preform can be made where cladding glass comprises a smallernumber of the high refractive index rods while core comprises a largernumber of high refractive index rods.

Conventional host for ytterbium in fiber lasers and amplifiers has beensilica glass host. Some level of aluminum or phosphorus is often addedto reduce ytterbium-clustering at high doping levels.Ytterbium-clustering is not desirable due to the fact that interactionbetween ytterbium ions could lead to multi-photon up-conversion andconsequently additional energy loss in a laser or an amplifier.

Additionally, photo-darkening is a phenomenon where background loss in afiber is permanently increased by creation of color centers as aconsequence of large amount of optical power's presence in the fiber.Photo-darkening is generally believed to be linked to ytterbiumclustering at high ytterbium doping levels, where multiple ions interactto produce photons with very high energy level to cause photo-darkening.Although the effect can be saturated after a period of exposure, itcontributes to loss of output power and reduction of efficiency in fiberlaser and amplifiers. Photo-darkening is more severe at high powerlevel; it can contribute to a significant power loss in a high powerfiber laser system if not dealt with appropriately.

Adding aluminum and phosphorus to reduce ytterbium clustering, however,has the effect of raising refractive index. Small amounts ofgermanium-doping can be added to raise refractive index if required.Fluorine can be added to lower refractive index. Due to the limitedamount of fluorine which can be incorporated into silica glass bycurrent state of art silica fiber fabrication techniques, rare earthdoped core typically has refractive index higher than that of thesilica. This is especially true for highly rare earth doped core, sincethe required doping level for aluminum or phosphorus is higher toachieve a reasonably low level of clustering, in addition to the indexincrease from rare-earth-doping. Accordingly, producing highly rareearth doped core glass, which is used for the power scaling of fiberlasers in the certain large core fiber designs, that have a refractiveindex close to that of the silica, is particularly difficult.

Various embodiments described herein, however, include highly rare earthdoped glass compositions having a refractive index close to that ofsilica. Such glasses can be fabricated into rods that can be used aperforms to produce other rods as well as optical fiber. Moreover, suchglasses can be fabricated with mature technologies used in themanufacture of optical fibers in telecommunications industry.

One embodiment shown in FIG. 1, for example, comprises a glass rod orperform 2 comprising a glass composition with high levels of rare earthions, e.g. >1000 mol ppm, with refractive index very close to that ofthe silica glass, e.g. index difference Δn<1×10⁻³. In this embodiment,the mostly silica glass has significant levels of phosphorus, e.g.,10-50 mol % phosphorus, and boron, e.g. 8-50 mol % boron. In variousembodiments, the phosphorus and boron are in a phosphorus or boroncontaining compound. The mostly silica glass may, for example, comprises5-25 mol % P₂O₅, and 4-25 mol % B₂O₃. An elemental analysis, e.g., usinga mass spectrometer or SEM, may be used to determine glass composition.It has been found that such glass compositions allow ytterbium dopinglevels up to 20,000 mol ppm without devitrification and enable highlyefficient laser and amplifier actions. It has been discovered thatphoto-darkening effect is substantially lower in the fabricated fibersfor equivalent amount of ytterbium doping level. The use of this glasscan reduce photo-darkening and lead to much more stable and efficienthigh power fiber lasers and amplifiers.

Various embodiments described herein also include techniques forfabricating doped fiber core with extremely high effective refractiveindex uniformity. This high uniformity can be achieved by fabricating alarge number of rare earth doped performs having core and cladding. Theaverage refractive index of the doped core glass is fabricated to bevery close to that of the silica glass. The cladding glass of thepreforms may be totally or partially removed by grinding or drilling.The result can be a rod such as shown in FIG. 1. The resulting rods arestacked and the stack is drawn into a smaller size rod while ensuringsubstantial fusion of constituent rods in the stack. The stack canoptionally be inserted into a tube before drawing. The drawn rod can becut and re-stacked with other similar rods and drawn again following asimilar procedure. This process can be repeated many times to obtain ahomogenous rod at the end. In the final fiber, the doped glass may befurther mixed by flow and diffusion of the low viscosity rare earthdoped glass at the high drawing temperature. In the alternativearrangement where the stack is inserted into a tube, an additional stepmay be added to fuse the structure. Part or all of the tube may then beremoved by grinding or etching before drawing into rods.

In contrast to other doped glasses, this rare earth doped glassdescribed herein allows refractive index very close to that of thesilica glass to be made. Furthermore, highly uniform rare earth dopedcore glass can be made by repeated stack-and-draw of the same type ofglass rods instead of two types of glass rods used in other methods.This approach further improves glass uniformity and allows higheffective rare-earth-doping levels in the fiber core.

Various fibers and processes for fabricating fibers formed by stackingand canning performs are described in greater detail below. Processes offabricating the performs are also described. Processes for fabricatingperforms, fibers, and other structures will be described in detail byway of examples and illustrations. Wide variation in the processes andresultant and intermediate products, however, are possible. Processingsteps may be added, removed, or reordered. Similarly, components may beadded, removed, or configured differently or in different amounts. Inparticular, starting constituents and added components may be varied.

The preform fabrication technique used in the following examples ismodified chemical vapor deposition (MCVD), although other known vapordeposition techniques such as, e.g., outside deposition process can alsobe used. Silicon and phosphorus are introduced to the deposition zonethrough the conventional bubblers arrangement with liquid precursors,e.g., SiCl₄ and POCl₃ respectively. Boron is introduced into thedeposition zone through a gas precursor, e.g., BCl₃. Fluorine is alsoused and is introduced through CF₄. Rare earth ions are introducedthrough the well known solution-doping process.

At an early stage of the process, a substrate tube 10 of 25 mm outerdiameter and 19 mm inner diameter is cleaned and placed on a lathe of aMCVD system as illustrated in FIG. 2A. A standard MCVD systemmanufactured by Nextrom Technologies, Vantaa, Finland, for optical fibermanufacturing is used in this example, although other systems may beemployed. The substrate tube 10 is joined with a starting tube 11 ofsimilar diameter at a first end and a soot tube 12 of larger diameter ata second end. The starter tube 11 is held by chucks 13 and is connectedto gas inlet line 16 through rotational seal 15. Soot tube 12 is alsoheld by chuck 14. The soot tube 12 is connected to a scrubber whichprocesses the exhaust gas before releasing. A traveling burner 17 whichmoves along the substrate tube 10 can heat a portion of the substratetube at a time. When the burner 17 reaches the end of the substrate tube10 near the joint with the soot tube 12, it quickly moves to thebeginning of the substrate 10 near the joint with the starter tube 11.It will be ready for next pass along the substrate tube 10. The chucks13, 14 rotate at a rate of 40 rpm.

The substrate tube 10 is initially cleaned with an etching pass where200 sccm of CF₄ is used to remove a layer of glass from the innersurface of the substrate tube 10. One or more optional cladding passesare used to deposit a cladding with refractive index close to thesubstrate tube, which may comprise silica. A flow of 500 sccm SiCl₄, 300sccm POCl₃ and 10 sccm SF₆ may be used to form a cladding.

A core layer is deposited by moving the burner 17 upstream from the soottube end as illustrated in FIG. 2B. The burner 17 moves upstream aheadof the soot formation so that the burner will not go over the core soot20 to avoid any soot sintering as shown in FIG. 2B. In certainembodiments, the flow used for the core formation is 100 sccm SiCl₄, 500sccm POCl₃ and 200 sccm CF₄. During the core pass, the burner 17 travelsfrom the soot tube end to the starting tube end at 20 mm/min toencourage soot formation. For the burner, 47 slm of hydrogen andoxygen/hydrogen ratio of 0.45 are used. The core layer is then followedwith a pass to consolidate the core soot 20 by heating it to 1300° C.

The substrate tube 10 is then removed from the lathe to soak the coresoot 20 in YbCl₃ solution to incorporate ytterbium into the soot layers.After 1 hour of solution-doping, the solution is drained and thesubstrate tube 10 is placed back on the lathe. The substrate tube isdried for a few hours by passing nitrogen and heating the substrate tubeup to ˜1000° C. The core soot 20 is sintered by heating it to 1750° C.with a BCl₃ flow of 10-150 sccm. After the sintering pass, the substratetube 10 goes through a collapse process with some level of POCl₃over-doping, where the burner temperature is significantly raised andburner speed lowered. The surface tension of the tube reduces the tubeouter diameter during the collapse process. In some embodiments, 3-5collapse passes are used. The tube is totally collapsed into a rod atthe end. A solid preform with ytterbium-doped core is obtained. In thisembodiment, the boron and phosphorous are introduced at different timesthereby reducing the likelihood of the reaction of boron and phosphorousin vapor phase.

Curve 100 in FIG. 2C illustrates the refractive index profile over thecore part of a fabricated ytterbium-doped preform. The averagerefractive index of this preform over the core is shown by line 101along with the target refractive index of silica shown as line 102. Theaverage refractive index of the preform over the core shown as 101 canbe made to be within ±1×10⁻⁴ of the target refractive index shown as102. An example of the average refractive index made with conventionalsilica host is illustrated by line 103, which is much higher. Thesignificant increase in matching is produced by the much larger amountof B₂O₃ level incorporated in the glass, which lowered the refractiveindex of the doped glass.

The core is doped with 3500-17500 mol ppm of Yb³⁺ ions, givingabsorption of 300-1500 dB/m at 976 nm. In this example, the core isfurther doped with 15-25 mol % of P₂O₅, 0.1-0.5 mol % F and 10-25 mol %B₂O₃. For a 50 μm core fiber, in certain embodiments, the difference ofthe average refractive index and that of the silica, Δn=n−n_(silica), islarger than −5×10⁻³ and smaller than 5×10⁻⁴ at a wavelength of 1 μm. Thephoto-darkening effect is also substantially reduced in the fabricatedfibers for equivalent amount of ytterbium doping level possibly at leastin part because of the inclusion of phosphorus, which prevents ytterbiumclustering. Variation in the parameters used to fabricate the fiber maybe used and the results may also vary.

FIG. 3A illustrates the structure of a preform 200 comprising of a core201, an optional deposited cladding layer 202 and silica glass layer 203from the substrate tube 10 that may be fabricated in a process describedabove with reference to FIGS. 2A-2C. FIG. 3B further illustrates thecore rod 210 made from the preform 200 by removing silica layer 203,deposited cladding layer 202 and perhaps small part of the core 201.Cross-section 211 includes all or part of core 201. In an alternativeembodiment, rod 211 contains part of the cladding layer 202.

The repeated stack-and-draw process starts with fabrication of multiplepreforms with rare earth doped core. The preforms are then grinded toremove the silica layer 203 and totally or partially to remove depositedcladding layer 202 to produce multiple of core rods 210 comprises mostlyrare earth doped core glass. Etching away glass layers which do notcontain ytterbium and drilling out the doped core can be used for thispurpose as example alternatives.

The doped core rods 210 can then be stacked as illustrated in FIG. 4A.Hexagonal stack 300 shown in FIG. 4A may be used due to its high packingdensity, although other stacking arrangement can also be used. The twoends of the stack 300 are fused while the stack is held in place. Thefused ends can then hold the shape of the stack 300 during caning.Fusing the entire stack 300 is optional. The doped core rods 210 canalso be joined with undoped rods at one or both ends to reduce wastingdoped core rods in the subsequent caning process illustrated in FIG. 4A.The stack 300 can then be caned into a single rod 310 by choosingappropriate drawing conditions for fusing all the rods in the stack. Thestack 300 can also be inserted into a tube (not shown in FIG. 4A) beforedrawing. The tube can help holding stack 300 in place during drawing.The caned rod 310 has a substantially fused cross-section. FIG. 4B showa refractive index profile 320 across a cross-section along AA shown inFIG. 4A. The refractive index profile 320 comprises mainly therefractive index of the original core rods 210 scaled down in dimension.Flow and diffusion can occur at higher canning temperatures. Peaks andvalleys of the refractive profile of the original rods 210 can besubstantially smoothed out.

The cane 310 can then be cut into multiple sections and stacked again toform stack 400 for a repeated process illustrated in FIG. 5A. Thisprocess can be repeated many times. FIG. 5B shows a refractive indexprofile 420 along line BB of the resulting cane 410 of FIG. 5A. Therefractive index profile 420 comprises mainly the refractive indexprofiles of original canes 310 scaled down in dimension. Again, therefractive index profile 420 (viewed along BB) can be substantiallysmooth out by flow and diffusion if caning temperature is high. This cancreate a more uniform refractive profile. At least two stages are usedin some embodiments as the second stage can eliminate inconsistency inthe fabricated preforms. In the stack examples in FIGS. 4A and 5A, astack of 37 rods are used. Other stack sizes can also be used. Thenumber of rods used in each stage does not need to be the same. Thearrangement may also vary in different embodiments.

In another example of this process, the stack is inserted into a tubebefore drawing in each stage of a two-stage process. Additionally, anextra step can be used to fuse the stack and the tube prior to drawing.The tube may then be totally or partially removed by grinding oretching. Additionally, the doped rod is incorporated into a core regionof a large core fiber. Such a fiber may be fabricated by stacking thedoped core together with undoped rods as well as with hollow (undoped)rods and drawing. A cross-section of a fiber 500 thereby produced isshown in FIG. 6. The fiber 500 includes 6 holes 502, which defines acore 501. The 6 holes may be formed from six hollow rods used in thedrawing process. Other processes and configurations may be used. Thedoped part of the core 503 comprises the doped rods fabricated by therepeated stack-and-draw process such as described above with regard toFIGS. 4A and 5A, which shows first and second stacks, respectively. Eachof the rods used in the second stack are visible because the optionalsilica tube used in the first stack has slightly lower refractive indexthan that of the ytterbium-doped glass.

In the embodiment shown in FIG. 6, silica glass is used as the matrixfor the doped rods as well as for the holes 502 in the holey fiber 500.In various other embodiments, silica may be used as the matrix for dopedrods and/or for the holes in holey fiber. Accordingly silica may be usedas a matrix for the doped regions in the core 503 and may also be usedas matrix material for creating the cladding. Other materials may alsobe used as the matrix material in other embodiments. Additionally,different matrix materials may be used for any of the core or claddingregions, for example, to surround the doped regions (e.g., rods) or theholes in holey fiber.

FIGS. 7A-7D illustrates various embodiments where the doped core rodsare chosen to have a different average refractive index. FIG. 7Aillustrates a leakage channel fiber 500 with 6 holes 502, which form acore 501. Part 503 of core 501 is made from the doped rods. FIGS. 7B,7C, and 7D are illustrated refractive index profiles along line 504 inFIG. 7A where the doped core has an average refractive index equal to,higher than and lower, respectively, than refractive index of the silicaglass used.

FIG. 7B show a refractive index 611 of the doped part 503 of the core501 with an average index 612 that is matched to that of the silicaglass 610. The refractive index 613 of air is also shown in FIG. 7B.FIG. 7C shows the refractive index 621 of the doped part 503 of the core501 with an average index 622 that is higher than that of the silicaglass 610. In this case, there will be an additional waveguide due tothis high average refractive index. In certain embodiments, thedifference of the average refractive index and that of the silica,Δn=n−n_(silica), is small enough so that V=2πρNA/λ is kept below about6, where λ is optical wavelength, ρ is core radius and NA=(n²−n_(silica)²)^(1/2)≈n_(silica)(2Δ/n_(silica))^(1/2). In various embodiments, V isbelow about 2.4 so that higher order modes are not supported in thisadditional waveguide. In the case where this additional waveguide iscreated by the localize increase index, holes 502 can be reduced toreduce overall waveguide effect to reduce higher order mode propagation.The gap between holes provide leakage channels. Either hole size or/andnumber can be reduced to increase leakage. Higher order modes canthereby be leaked and few or single mode fiber provided.

FIG. 7D shows the refractive index 631 of the doped part 503 of the core501 with an average index 632 that is lower than that of the silicaglass 610. In this case, there will be an additional negative waveguidedue to this low average refractive index. In various embodiments, thedifference of the average refractive index and that of the silica,Δn=n−n_(silica), is larger than −0.005 so that it does not cancel thewaveguide effect from holes 502, may be larger than −0.001. In the casewhere this additional negative waveguide is present, holes 502 can beincreased to increase overall waveguide effect. Either hole size ornumber may be increased.

FIG. 8 illustrates a cross-section of a polarization maintaining (PM)fiber 700 incorporating two stress elements 704, in addition to 4 holes702. The core 701 comprises a doped portion 703 formed using doped rodssuch as those fabricated using processes described with reference toFIGS. 4A and 5A.

In the embodiment shown in FIG. 8, silica glass is used as the matrixfor the doped rods, the two stress elements 704 as well as for the holes702 in the holey fiber 500. Accordingly, in various other embodiments,silica may be used as the matrix for doped rods, stress elements, and/orfor the holes in holey fiber. Silica may be used as a matrix for thedoped regions in the core 703 and may also be used as matrix materialfor creating the cladding. Other materials may also be used as thematrix material in other embodiments. Additionally, different materialsmay be used in any of the core or cladding regions, for example, as thematrix for the doped regions (e.g., rods), the holes in holey fiber,stress elements, or other components.

Accordingly, although silica glass and air holes are used in the aboveexamples, many different transparent optical media with differentrefractive indexes can be used to implement the design. For an example,a soft glass can replace silica and a different soft glass with a lowerrefractive index can be used to replace the air holes. Rare earth ionsother than ytterbium can also be used. Still other variations, forexample, in configuration, materials, dimensions, or in other designparameters are possible.

Undesirable cracks can occur during cleaving due to the presence of thelarge holes. Cracks can make a cleaved end face unsuitable for use. Inaddition, for various applications, there is a need to seal the holes atfiber ends, for example, to prevent contaminants from entering theholes. A number of techniques can be used to resolve these issues. Inone embodiment shown in FIG. 9A, a coreless fiber 802 is spliced to alarge core holey fiber 801. The coreless fiber 802 is then cleaved toform the end facet 810. The cross-section of the cleaved end facet 810is shown in FIG. 9B. Splice point 804 and holes 803 are clearly shown inFIG. 9A. The coreless fiber may also serve as an end cap for beamexpansion if placed at the output of an amplifier. The end cap providesa uniform media where the beam can expand before exiting the glass. Thisbeam expansion can advantageously reduce optical intensity at theglass/air interface to reduce or minimize end face damage.

In another approach, holes 901 in a holey fiber 900 are totallycollapsed into a solid fiber 903 by heating a section of the holey fiberto a temperature at least as high as the glass softening temperature.The fiber 900 can then be cleaved in the collapsed portion to form anend facet 910 such as shown in FIG. 10A. FIG. 10B illustrates across-section of the collapsed and cleaved end facet 910 of a PM fiber.Stress elements 911 are visible on the fiber cross-section illustratedin FIG. 10B. Doped rods such as described above are also visible in thedoped section 912 of the core.

FIG. 10C illustrates a further technique where a holey fiber 920 withholes 921 is tapered and then collapsed. Drawing and/or heat can be usedto create the taper and collapse the holes 921, although the processshould not be so limited. The holey fiber 920 has a taper 923 and acleaved and tapered end 922. The end facet 922 in this case is shown inFIG. 10D. This taper 923 supports smaller number of modes and canfurther help increase transmission loss for higher order modes,especially when placed at the input end of an amplifier. Such inputtaper can also help efficiently launch optical power into the core.

The repeated stack-and-draw process described herein for making a dopedcore with extremely high effective refractive index uniformity can alsobe used in conventional step index fiber as well as photonic crystalfiber. In various embodiments wherein repeated stack and draw to producedoped core regions in conventional step index fibers, the averagerefractive index of the doped preform core can be made to be slightlyhigher than that of the cladding glass. The doped perform may comprise,for example, doped rods that form part of the core of the fiber. Incertain embodiments, the final rod made from the plurality of doped rodsis inserted into a tube comprising the cladding glass.

A fiber 1000 is shown in FIG. 11, where the doped core 1001 is made fromthe repeated stack-and-draw process. As described above, the doped core1001 may be formed from a plurality of doped rods. A cladding layer 1003comprises cladding glass. The fiber 1000 is further coated with acoating 1002.

A refractive index profile 1004 shows the refractive index differencebetween the doped core and cladding glass, Δn. The doped core havingvery high effective refractive index uniformity can improve mode qualityand ease of use and can also allow smaller Δn to be used. In manycurrent designs, Δn is chosen to be larger than 1×10⁻³, affected to someextent by the refractive index uniformity, repeatability of thefabrication process, and fundamental mode bend loss. Smaller Δn willallow larger core diameter to be implemented. For example, mode qualityin a 30 μm core fiber can be achieved in a 50 μm core fiber if Δn isreduced by a factor of (50/30)²≈2.8. Since bending loss is dependent onΔn, higher bending loss is expected for lower Δn for the same core size.In certain embodiments, single mode fiber is possible at ˜1 μmwavelength for 50 μm core diameter if Δn can be reduced to below 8×10⁻⁵,e.g. a NA≦0.015, with a compromise of bending performance. However,values outside these ranges are also possible.

As shown in FIG. 12, Stress rods 1104 can be included in the cladding1103 of a fiber 1100 to make large core polarization maintaining (PM)fiber with a highly uniform doped core 1101. An additional claddinglayer with lower refractive index than that of 1103 can also be addedbetween cladding 1103 and coating 1102 to form a pump guide in a doublecladding fiber structure. Coating 1102 can also be chosen to be of lowrefractive index to form a double cladding structure. It is alsopossible to dope only part of the core 1101 and use a similarstack-and-draw technique to make the rest of the core with a highlyuniform refractive index. Such selective doping of part of the core canbe used to further improve mode selection in a fiber.

As described above, smaller Δn in a conventional fiber allow larger corediameter to be used. However, higher bending loss is expected for lowerΔn. Various embodiments, described herein also provide designs forreducing this bending loss.

For example, the design in FIG. 13 can be used to improve (e.g., reduce)bending loss of large core fiber 1200 with very small NA, e.g. NA<0.05and weak guiding. A first cladding layer 1202 is placed next to a core1201 to provide the small Δn, which supports 1 to 10 modes in thepartially or entirely rare earth doped core. The index differencebetween the first layer 1202 and a second layer 1203, Δn₁, can be chosento be much larger than Δn, e.g. Δn₁>1.5Δn to reduce bend loss, helped bythe higher Δn₁. The fiber further comprises a coating 1204 surroundingthe second cladding layer 1203. The first cladding layer 1202 diameter,2ρ₁, only needs to be slightly larger than core 1201 diameter, 2ρe.g.ρ₁>1.1ρ. Too large ρ₁ and Δn can lead to large number mode beingsupported in the combined waveguide formed by core 1201 and the firstcladding layer 1202. Launching the fundamental mode may be difficult andinter-mode coupling may be increased if too many modes are supported bythe core 1201 and first cladding 1202. Since the mode supported in thecore 1201 has better overlap with the rare earth doped core andconsequently have more gain, light in modes with significant amount ofpower in the first cladding 1202 will be discriminated.

A double cladding fiber 1300 is shown in FIG. 14 like the fiber indepicted in FIG. 13, includes a core 1301, first cladding 1302 andsecond cladding 1303 and a coating 1304. The fiber shown in FIG. 14further comprises third cladding layer 1305 between a second claddinglayer 1303 and the coating 1304.

A small Δn may be provided between the first cladding layer 1302 and thecore 1301 to supports 1 to 10 modes in the partially or entirely rareearth doped core. The index difference between the first layer 1302 anda second layer 1303, Δn₁, can be chosen to be much larger than Δn, e.g.Δn₁>1.5Δn to reduce bend loss, helped by the higher Δn₁. The firstcladding layer 1302 diameter, 2ρ₁, only needs to be slightly larger thancore 1301 diameter, 2ρ, e.g. ρ₁>1.1ρ. Too large ρ₁ and Δn can lead tolarge number mode being supported in the combined waveguide formed bycore 1301 and the first cladding layer 1302. Launching the fundamentalmode may be difficult and inter-mode coupling may be increased if toomany modes are supported by the core 1201 and first cladding 1202.

The index difference between the second layer 1303 and a third layer1305 can be chosen to be much larger than Δn as well as well as largerthan Δn₁ in some embodiments. The third cladding 1305 can provide apropagation region comprising the core 1301, first cladding 1302, andsecond cladding 1303 for propagating pump radiation, for example, in afiber amplifier or laser.

The fibers 1200 and 1300 may have large effective area and may be dopedwith rare earth elements. Glass compositions having an index ofrefraction close to that of silica may also be used in the core asdescribed above. Additionally, stack and draw processes such asdescribed above may be used to provide increase uniformity in refractiveindex across the core. Other features and methods described herein mayalso be used in the fibers 1200, 1300 shown in FIGS. 13 and 14.

A wide variety of variations are possible. Components may be added,removed, or reordered. Different components may be substituted out. Thearrangement and configuration may be different. Similarly, processingsteps may be added or removed, or reordered.

While certain embodiments of the inventions have been described, theseembodiments have been presented by way of example only, and are notintended to limit the scope of the inventions. Indeed, the novel methodsand systems described herein may be embodied in a variety of otherforms; furthermore, various omissions, substitutions and changes in theform of the methods and systems described herein may be made withoutdeparting from the spirit of the inventions. The accompanying claims andtheir equivalents are intended to cover such forms or modifications aswould fall within the scope and spirit of the inventions.

What is claimed is:
 1. A large-core optical fiber comprising: a corehaving a core radius ρ and a core index of refraction n_(core), whereinsaid core comprises a doped region having an area of at least 200 μmsquare; a first cladding disposed about said core, said first claddinghaving an outer radius ρ₁ and an index of refraction n_(c1), said coreand said first cladding having a difference in index of refractionΔn=n_(core)−n_(c1); and a second cladding disposed about said firstcladding, said first cladding and said second cladding having adifference in index of refraction Δn₁, wherein the first claddingradius, ρ₁, is greater than about 1.1ρ and less than about 2ρ, and therefractive index difference between said first cladding and said secondcladding, Δn₁, is greater than about 1.5Δn and less than about 50Δn. 2.The large-core optical fiber of claim 1, said large-core optical fiberbeing bendable and configured with a reduced bend loss relative to alarge-core optical fiber having substantially the same core radius, ρ,and refractive index difference between cladding and core, Δn.
 3. Thelarge-core optical fiber of claim 1, wherein Δn is less than about 10⁻³.4. The large-core optical fiber of claim 1, wherein said core is aslarge as about 50 μm and said Δn is less than about 8×10⁻⁵.
 5. Thelarge-core optical fiber of claim 1, wherein said core is as large asabout 50 μm.
 6. The large-core optical fiber of claim 1, wherein saidlarge-core optical fiber comprises a step-index fiber.
 7. The large-coreoptical fiber of claim 1, wherein said first cladding has a numericalaperture (NA) less than about 0.05, said NA determined by n_(core) andn_(c1).
 8. The large-core optical fiber of claim 1, wherein said corecomprises a doped region having an average refractive index within about±0.003 or less of the refractive index of silica.
 9. The large-coreoptical fiber of claim 1, wherein said first cladding is slightly largerthan said core and said first cladding radius, ρ₁, is in the range1.1ρ<ρ₁<1.5ρ.
 10. The large-core optical fiber of claim 1, wherein1.5Δn<Δn₁<10Δn.
 11. The large-core optical fiber of claim 1, whereinsaid core is partially rare earth doped.
 12. The large-core opticalfiber of claim 1, wherein said core is entirely rare earth doped. 13.The large-core optical fiber of claim 1, wherein said core radius is aslarge as about 25 μm.
 14. The large-core optical fiber of claim 1,further comprising a third cladding disposed about said second cladding,and a pump propagation region comprising said core, said first cladding,and said second cladding.
 15. The large-core optical fiber of claim 1,further comprising a coating, and an additional cladding disposedbetween said coating and said second cladding to form a pump guide. 16.The large-core optical fiber of claim 1, wherein said core has aneffective refractive index uniformity with less than about 5×10⁻⁴maximum peak-to-peak variation.
 17. The large-core optical fiber ofclaim 1, wherein said large-core optical fiber is polarizationmaintaining.
 18. The large-core optical fiber of claim 1, wherein saidsecond cladding comprises holes that are configured to provide leakagechannels such that said large-core optical fiber supports one or a fewmodes and higher order modes are leaked.
 19. The large-core opticalfiber of claim 1, wherein a portion of said large-core optical fibercomprises holey fiber.
 20. The large-core optical fiber of claim 1,wherein said core comprises: silica having a refractive index; at leastabout 10 mol % phosphorus in said silica; at least about 10 mol % boronin said silica; rare earth ions in said silica, said rare earth ionshaving a concentration in said silica of at least about 1000 mol ppm,wherein said silica having said phosphorus, said boron, and said rareearth ions therein has a refractive index within about ±0.003 or less ofthe refractive index of the silica.
 21. An optical fiber system forproviding optical amplification, the optical fiber system comprising:the large-core optical fiber of claim 1, wherein said core of saidlarge-core fiber is doped with one or more types of rare earth ions,said large-core optical fiber comprising a combined waveguide formed bysaid core and said first cladding layer; an optical pump opticallycoupled to said large-core optical fiber; and an optical sourceoptically coupled to an input of said large-core optical fiber.
 22. Theoptical fiber system of claim 21, wherein said combined waveguide isconfigured such that a mode supported in said core has increased gainrelative to a mode having substantial power in said first cladding. 23.The optical fiber system of claim 21, wherein said core is configured toreceive an input beam launched in said core, wherein said input beam isin a fundamental mode of said large-core optical fiber.
 24. The opticalfiber system of claim 21, wherein said large-core optical fiber isconfigured to support 1 to 10 modes in said core.
 25. The optical fibersystem of claim 21, wherein values of said first cladding radius ρ₁ andsaid Δn are such that the number of modes supported in said combinedwaveguide and inter-mode coupling among the number of supported modesare limited such that a mode supported in said core has increased gainrelative to a mode having substantial power in said first cladding. 26.The optical fiber system of claim 21, said core having substantialrefractive index uniformity.
 27. The optical fiber system of claim 21,wherein said core radius is as large as about 25 μm.
 28. The opticalfiber system of claim 21, wherein a portion of said large-core opticalfiber comprises holey fiber.
 29. The optical fiber system of claim 21,wherein said large-core optical fiber is bendable and configured with areduced bend loss relative to a large-core optical fiber havingsubstantially the same core radius, ρ, and refractive index differencebetween cladding and core, Δn.
 30. The optical fiber system of claim 21,wherein said large-core optical fiber further comprises a third claddingdisposed about said second cladding, and a pump propagation regioncomprising said core, said first cladding, and said second cladding. 31.The large-core optical fiber of claim 21, further comprising a coating,and an additional cladding disposed between said coating and said secondcladding to form a pump guide.
 32. The optical fiber system of claim 21,wherein said first cladding is slightly larger than said core and saidfirst cladding radius, ρ₁, is in the range 1.1ρ<ρ₁<1.5ρ.
 33. The opticalfiber system of claim 21, wherein said input of said large-core opticalfiber is tapered, said large-core optical fiber has a length extendingfrom said tapered input, and said tapered input supports a reducednumber of optical modes than the length extending from said taperedinput.
 34. The optical fiber system of claim 21, wherein said core ofsaid large-core optical fiber comprises: silica having a refractiveindex; at least about 10 mol % phosphorus in said silica; at least about10 mol % boron in said silica; rare earth ions in said silica, said rareearth ions having a concentration in said silica of at least about 1000mol ppm, wherein said silica having said phosphorus, said boron, andsaid rare earth ions therein has a refractive index within about ±0.003or less of the refractive index of the silica.
 35. The optical fibersystem of claim 34, wherein said large-core optical fiber has a pumpabsorption in a range from 300 to 1500 dB/m.